1. Field of the Invention
This invention relates in general to measurement of pressure fluctuations in a high speed boundary layer along a test surface, and specifically to a method for making a stretched membrane in a dynamic pressure sensor fit precisely flush with a test surface so fluid flow along the test surface is not disturbed by the membrane or its edges.
2. Description of the Related Art
In a known pressure sensor, the sensing element is a thin stretched membrane, which is displaced from its equilibrium position in proportion to an incident time-varying pressure. The deflections of the membrane can be measured by several known transducer methods, for instance by electronic measurement of variations in capacitance between the membrane and a fixed electrode. A stretched membrane pressure sensor has the advantages of superior acoustic impedance match to air and low surface mass density, so it provides high sensitivity to applied pressure variations and low sensitivity to mechanical vibrations.
A stretched membrane can be made by starting with an oversized flat membrane that is prestressed in a fixture while the center of the membrane is bonded to the top surface of the pressure sensor body. After the membrane is bonded to the pressure sensor body, the pre-stress fixture is removed, and the edges of the membrane are cut flush with the edges of the pressure sensor body. If the bond is by use of an adhesive, it is possible to get a flat membrane that can be fitted flush with a test surface through a hole in the test surface. This design has the disadvantage that the dynamic characteristics of the pressure sensor depend severely upon the strength of the adhesive bond, which deteriorates with aging and temperature cycling and is subject to creep. Bonding a prestressed flat membrane by brazing or welding has been attempted, but it has proven difficult to obtain an accurately flat membrane when such bonding methods are used.
A stretched membrane can also be made by stretching a slack membrane after it is mounted on the pressure sensor body. A known design starts with a cup-shaped membrane, which is fitted over the top of a tubular pressure sensor body, whereafter the side wall of the cup-shaped membrane is brazed or welded to the side wall of the pressure sensor body. Finally, a thin-walled tube fitting inside the tubular pressure sensor body is pressed against the inside of the membrane to produce the desired pre-stress in the membrane. This design provides stretched membrane with an accurately flat top surface, but the membrane will always have a rounded edge, so the flat membrane surface cannot extend all the way to the outside wall of the pressure sensor body. This design accordingly causes a gap between the edge of the mounting opening and the flat part of the membrane, which destroys the continuity of the test surface.
In cases where pressure variations in a high speed boundary layer along a test surface are to be measured, it is imperative that surface discontinuities resulting from pressure sensor mounting do not exceed the roughness of the test surface. Even minute pressure sensor misalignment can lead to measurement errors, premature transition to turbulence, or other disruptions of the flow. None of the known pressure sensors can provide both perfectly flush mounting and freedom from creep and aging in the measurement accuracy.